Low-electrical resistivity polycrystalline TiO2-based transparent conductors by millisecond flash lamp annealing of magnetron sputtered films

Transparent conductive oxides (TCO), mainly In2O3:Sn (ITO), ZnO:Al (AZO) and SnO2:F (FTO) are widely used as transparent electrodes in flat panel displays, thin film solar cells and solid state lighting. In contrast to these TCOs, TiO2 –based films offer unique combination of low cost, high refractive index, stability against humidity, the high chemical stability and the non-toxicity. The Nb or Ta doped TiO2 films epitaxially grown on crystalline substrates already show electrical and optical properties which are comparable to those of conventional TCOs. However, it is still a challenge to achieve low electrical resistivity polycrystalline TiO2 - based films as required for most of applications. Furthermore, it is not possible to get low resistivity in polycrystalline films by direct growth at elevated substrate temperatures. Only a two-step approach, i.e. the deposition of amorphous films followed by annealing for minutes up to hours in vacuum or hydrogen delivers films with resistivity values in the range of 1•10-3 Ωcm. Both, the direct growth on crystalline substrates and the post deposition annealing of the amorphous films require substrate temperatures of about 400°C to ensure desired resistivity, which drastically limits applications. Furthermore, the high demand for energy for an annealing time of several minutes or even hour is almost unacceptable for a cost-efficient and environmental friendly replacement of conventional by TiO2 - based TCOs, especially for large area applications. In order to address this problem, we studied the films formed on glass substrates without heating by direct current magnetron sputtering (DC MS) of reduced TiO2:Ta ceramic targets followed by flash lamp annealing (FLA) in the millisecond range to crystallize as-deposited amorphous films. The Ti/O ratio of the as-deposited films was varied using a DC MS process in conjunction with a plasma feedback system to achieve an oxygen fine-tuning. Using FLA, the heat treatment is confined to the film and the substrate is only partly heated (several µm at the surface) which drastically lowers the energy consumption and allows the use of temperature sensitive substrates. In addition, the short annealing enables the film processing at atmospheric pressure in argon or even in air. The emerging electrical, optical and structural properties are strongly affected by the Ti/O ratio in the as-deposited films adjusted by the plasma feeback system Our approach delivered films with an electrical resistivity in the range of 1•10-3 Ωcm, optical transmittance above 80% for 400nm thick films and electrical activation of Ta dopants up to 60%. The reference films obtained combining deposition onto unheated substrate and subsequent conventional annealing in vacuum at 425°C for 1 hour show almost the same electrical and optical properties.